Re: [PATCH net-next] Revert "net/ipv6: add sysctl option accept_ra_min_hop_limit"

[Sabrina Dubroca <sd@queasysnail.net>]

2015-09-02, 16:11:10 -0700, David Miller wrote:
> From: Sabrina Dubroca <sd@queasysnail.net>
> Date: Wed, 2 Sep 2015 11:43:01 +0200
> 
> > This reverts commit 8013d1d7eafb0589ca766db6b74026f76b7f5cb4.
> > 
> > There are several issues with this patch.
> > It completely cancels the security changes introduced by 6fd99094de2b
> > ("ipv6: Don't reduce hop limit for an interface").
> > The current default value (min hop limit = 1) can result in the same
> > denial of service that 6fd99094de2b prevents, but it is hard to define
> > a correct and sane default value.
> > More generally, it is yet another IPv6 sysctl, and we already have too
> > many.
> > 
> > This was introduced to satisfy a TAHI test case which, in my opinion, is
> > too strict, turning the RFC's "SHOULD" into a "MUST":
> > 
> >     If the received Cur Hop Limit value is non-zero, the host
> >     SHOULD set its CurHopLimit variable to the received value.
> > 
> > The behavior of this sysctl is wrong in multiple ways.  Some are
> > fixable, but let's not rush this commit into mainline, and revert this
> > while we still can, then we can come up with a better solution.
> > 
> > Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
> 
> I don't agree with this revert.
> 
> If you look at the original commit, the quoted RFC recommends adding
> a configurable method to protect against this.
> 
> And that's exactly what the commit you are trying to revert is doing.
> 
> The only thing I would entertain is potentially an adjustment of the
> default, working in concert with the TAHI folks to make sure their
> tests still pass with any new default.

Would you agree with a default of 64, as Florian suggested?


Can we still modify the behavior of this sysctl? It's already been in
Linus's tree for a while, but if we can, I would rather restrict the
values we let the user write to accept_ra_min_hop_limit, as anything
outside [0..255] does not really make sense.

Allowing an RA to update the hop limit if

   current hop limit < RA.hop_limit < accept_ra_min_hop_limit

might also be desirable, but I'm not so sure about this case.


-- 
Sabrina